Mule deer (Odocoileus hemionus) fawn:doe (f:d) ratios in December have declined by 0.015 fawns/doe/year from 1972 through 1995 in Colorado, USA. To determine whether lower pregnancy or fetal production was the cause of low December f:d ratios, we estimated mule deer pregnancy rates in the Poudre River drainage during January 1998 and pregnancy and fetal rates on the Uncompahgre Plateau during February 1999. Twenty-seven (93.1%) of 29 yearling and adult does in the Poudre River drainage were detected pregnant with pregnancy-specific protein-B (PSPB). This proportion did not differ (P > 0.2) from the proportion of yearling and adult does that were pregnant when collected by various methods in previous studies in Colorado. Thirty-six (90%) of 40 adult does were detected pregnant with PSPB, and 37 (93%) of these 40 does on the Uncompahgre Plateau were detected pregnant with ultrasound. Proportion of adult does detected pregnant with ultrasound did not differ (P > 0.07) from pregnancy rate of adult does collected by various methods in previous studies. Average number of fetuses per doe for all adult does (pregnant and nonpregnant) on the Uncompahgre Plateau (n = 40, x̄ = 1.70, SE = 0.109) was not less (P > 0.06) than for adult does collected by various methods in previous studies. We concluded that failure to breed or maintain pregnancy through at least early January or early February does not appear to be the cause of low f:d ratios observed in Colorado.